1. Field of the Invention
The present invention relates to an image forming apparatus that scans light irradiated on a photosensitive member while rotating the photosensitive member, thereby forming an image on the photosensitive member.
2. Description of the Related Art
In recent years, electrophotographic printers have been increasing in performance, and techniques for realizing improvements in responsiveness in printing, print speed, and print image quality, and low cost.
Examples of indexes for evaluating the responsiveness include FPOT (First Print Out Time) and FCOT (First Copy Out Time) as the time period between user's print instruction and the completion of output a first recording medium with an image formed thereon. FPOT and FCOT are desired to be a few seconds or less.
By the way, a thickness of a photosensitive layer (hereinafter referred to as the “film thickness”) of a photosensitive drum in electrophotographic printers cannot be uniform due to the limits of production accuracy. Moreover, a surface of the photosensitive drum wears because the photosensitive drum comes into contact with a recording medium, an intermediate transfer member, or a cleaning member during the formation of an image. At this time, the wear amount differs at individual positions of the photosensitive drum, and hence the unevenness of the film thickness is further promoted. In a case where such a photosensitive drum is charged and exposed to light, potential characteristics of the surface of the photosensitive drum cannot be uniform. For this reason, there are variations in an image density of an output image. Therefore, to correct for variations in the image density of an output image resulting from nonuniform potential characteristics of the surface of the photosensitive drum and improve image quality, a technique to correct for variations in the potential characteristics of the surface of the photosensitive drum has been developed. As examples of such a technique, those disclosed in Japanese Laid-Open Patent Publication (Kokai) No. S63-49779, Japanese Laid-Open Patent Publication (Kokai) No. 2004-223716, Japanese Laid-Open Patent Publication (Kokai) No. 2007-187829, and Japanese Laid-Open Patent Publication (Kokai) No. 2007-34233 are known. These prior arts are a technique to correct for variations in the potential characteristics of the photosensitive drum by adjusting a laser light exposure amount according to a position exposed to light when an exposure unit exposes the photosensitive drum to light. This technique have realized an improvement in print quality, resulting in an improvement in allowable variation level, and a decrease in the production cost of the photosensitive drum which is highly-durable and long-lived and allows variations in the potential characteristics.
FIG. 6 is a diagram schematically showing an arrangement of an optical writing unit in an image forming apparatus that corrects for variations in the potential characteristics of the surface of a photosensitive drum. The image forming apparatus is a one-photosensitive drum four-beam laser simultaneous scanning electrophotographic printer. For the simplification of explanation, units for forming an image on a paper medium, such as a charging unit, a developing unit, a transfer unit, and a fixing unit associated with a general electrophotographic process are omitted from the figure.
The image forming apparatus has a multi-beam laser optical writing unit 1100, a system control unit 1140, an image data process unit 1150, a photosensitive drum 1130, and so on. The system control unit 1140 controls the overall operation of the apparatus, and is comprised of a CPU, a ROM, a RAM, a user interface (not shown) for controlling devices, and so on.
The image data process unit 1150 is comprised of an ASIC, and operates while communicating information with the system control unit 1140 by register access from the CPU of the system control unit 1140. A drum drive unit 1136 that drives the photosensitive drum 1130 to rotate is provided on a side of the photosensitive drum 1130.
FIG. 7 is a timing chart showing operations of the units when the photosensitive drum starts rotating in the image forming apparatus. The timing chart shows a photosensitive drum drive state 1201, a photosensitive drum potential characteristic variation correcting process valid/invalid state 1202, and a rotation reference position signal 1122. The rotation reference position signal 1122 is obtained by a rotation reference position sensor 1120 and a rotation position mark 1121. States a to e in the figure correspond to determinations about control states by the system control unit 1140 and state transitions responsive to control instructions.
FIGS. 8A and 82 are flowcharts showing procedures of an image forming operation and procedures of a potential characteristic variation correcting operation. In the figure, FIG. 8A shows the operation of the CPU in the system control unit 1140, and FIG. 8B shows the operation of a potential characteristic correction unit 1161 in the image data process unit 1150 (ASIC). In the figure, broken lines indicate transmission of information via input/output signals to and from the devices.
Referring to FIGS. 6, 7, 8A, and 8B, a description will be given of the operation of the image forming apparatus. The system control unit 1140 brings the image forming apparatus into an image formation stopped-and-drum stopped state as an initial state (step S101). This state is represented by the drive state a in FIG. 7. On the other hand, the potential characteristic correction unit 1161 lies in a state of not corrected for variations as an initial state. Namely, the potential characteristic correction unit 1161 is waiting in the variation correcting process invalid state d in FIG. 7.
The system control unit 1140 waits for an instruction to start image formation (step S102). Upon receiving the instruction to start image formation, the system control unit 1140 instructs the drum motor drive unit 1136 and a polygon mirror motor drive unit 1103a to start operating, thus starting a preparation for image formation (step S103). In the preparation for image formation, the drum drive unit 1136 starts driving the photosensitive drum 130 according to a rotation instruction signal 1141 given to the drum drive unit 1136. At the same time, the polygon mirror motor drive unit 1103a starts rotating a polygon mirror 1103 at constant speed according to a rotation instruction signal 1142 for scanning laser light. At this time, the drive state 1201 of the photosensitive drum 1130 is the drive state b.
The system control unit 1140 waits until a predetermined waiting time period has elapsed after the start of the motors (step S104). When the predetermined waiting time period has elapsed, the drive state 1201 of the photosensitive drum 1130 changes to the drive state c in which the photosensitive drum 1130 rotates at stable constant speed required for image formation.
The system control unit 1140 confirms an input of the rotation reference position signal (drum reference signal) 1122 (step S105). In the drive state b in which the photosensitive drum 1130 starts rotating and the drive state c, the rotation reference position signal 1122 is generated each time the rotation position mark 1121 on the photosensitive drum 1130 passes the rotation reference position sensor 1120 of the photosensitive drum 1130.
After the drive state 1201 of the photosensitive drum 1130 changes to the drive state c in which the photosensitive drum 130 rotates at stable speed, the rotation speed of the polygon mirror 1103 stabilizes, and further, at the time when the rotation reference position signal 1122 is inputted, the system control unit 1140 instructs to correct for variations in potential characteristics (step S106). As a result, a register access instruction 1143 for the correction for variations in potential characteristics is generated, and the variation correcting process valid/invalid state 1202 changes to the valid state e.
The system control unit 1140 carries out the image forming operation (step S107). During the image forming operation, the image data process unit 1150 starts image data process in response to the register access instruction 1143 for image formation from the system control unit 1140.
When image data 1151 is inputted from an external personal computer (not shown), the image data 1151 is sent to a line buffer control unit 1152 and stored as data in line buffers 1153 corresponding in number to the number of multiple lasers. The stored data is read in parallel as data corresponding in number to the number of multiple lasers from the line buffers 1153 in timing with BD signals 1135, and sent to a photosensitive drum potential characteristic correction image process unit (multiplication unit) 1154.
Next, a description will be given of a flow of a photosensitive drum potential characteristic variation correction image process. After the photosensitive drum 1103 goes into the drive state c in which it rotates at stable speed, the potential characteristic correction unit 1161 carries out an address computation at the time when the rotation reference position signal 1122 is inputted (step S201). The address computation is carried out based on the BD signal 1135 indicative of a scanning start position generated by laser light incident on a laser light sensor for controlling a beam exposure start position in laser scanning, and counting using crystal oscillator clocks. As a result of the address computation, an appropriate address for taking out variation correction data is selected.
The potential characteristic correction unit (memory controller) 1161 selects and successively reads correction data from a nonvolatile memory (table memory) 1160 storing table data on variations in photosensitive drum potential characteristics (correction data) (step S202). The correction data is prepared in advance in the table memory 1160 in accordance with a photosensitive drum provided in the image forming apparatus.
The potential characteristic correction unit 1161 causes matching process units 1163 to successively carry out processes for matching the table data on variations in photosensitive drum potential characteristics transmitted via memory data buses 1162 to data that is to be multiplied with image data (step S203). Further, the potential characteristic correction unit 1161 successively transmits the variation correction data from the matching process unit 1163 to the multiplication unit 1154 (step S204). The multiplication unit 1154 multiplies the image data with the variation correction data. After that, the potential characteristic correction unit 1161 terminates the present operation.
In the above described way, after the drive state 1201 of the photosensitive drum 1103 goes into the drive state c in which it rotates at stable speed, at the time when the rotation reference position signal 1122 is inputted, an appropriate address for taking out variation correction data is selected based on the BD signal 1135 indicative of a scanning start position and counting using the crystal oscillator clocks. Thereafter, the potential characteristic correction unit 1161 goes into a state of correcting for variations in potential characteristics, and the variation correction process valid/invalid state 1202 goes into the valid state e.
The data having been subjected to the potential characteristic variation correcting process by a computation (multiplication) of the data from the line buffers 1153 and the variation correction data in the multiplication unit 1154 is transmitted to a laser Pulse Width Modulation Unit (PWMU) 1155, and made available for use in blinking a four-beam multi-laser semiconductor chip 1101 via a laser current drive unit 1106.
The laser light is gathered by a collimator lens 102 and then reflected/scanned by the polygon mirror 1103 to pass through an fθ lens 104. Further, the laser light follows a laser light path 1105 from the polygon mirror 1103 to the photosensitive drum 1130 and is scanned on the photosensitive drum 1130 along paths taken by scanning lines 1133 of exposure spots 1131 by the four-beam multi laser by rotation of the polygon mirror 1103. On the photosensitive drum 1130 thus charged, an electrostatic latent image is formed.
After the electrostatic latent image is formed, the image forming apparatus develops the electrostatic latent image with toner, transfers the image to a paper medium, fixes the image on the paper medium by heating and pressurizing, so that an image is formed.
When a time period required to form an image of a predetermined size has elapsed, the system control unit 1140 determines whether or not the image forming operation has been completed (step S108). When the image forming operation has been completed, the system control unit 1140 outputs the rotation instruction signal 1141 to stop the rotation of the photosensitive drum 1103 (step S109), and waits until the stop of the photosensitive drum 1130 is confirmed (step S110). Upon confirming the stop, that is, the completion of the deceleration, the system control unit 1140 terminates the present operation.
By carrying out the above described operation, the image forming apparatus draws a latent image on which the correction for variations in the potential characteristics of the photosensitive drum has been carried out, and the correction for variations in laser writing has been carried out.
However, there are problems as described below in improving the performance of the conventional image forming apparatus. When acceleration control for accelerating the photosensitive member is switched to constant-speed control, the formation of an image may not be started immediately depending on the position of the rotation position mark 1121 provided on the photosensitive drum relative to the rotation reference position sensor 1120. The correction for variations in the potential characteristics of the photosensitive drum is started in response to generation of the rotation reference position signal (drum reference signal) 1122. The rotation reference position signal 1122 is generated in response to the rotation position mark 1121 provided on the photosensitive drum passing the rotation reference position sensor 1120, but the formation of an image cannot be started unless the rotation reference position signal 1122 is generated even when the formation of an image is ready to be started in a state in which the photosensitive drum is rotating at constant speed. For this reason, there may be a case where the formation of an image cannot be started until substantially one turn of the photosensitive drum is completed after the rotation speed comes to be a constant speed, and in this case, FPOT lowers.
For example, if the photosensitive drum of 80 mmφ is rotated at a surface speed of 251 mm/sec, the time period required for one turn of the photosensitive drum is as follows.80*3.14/251−251 mm/251=1 sec
In a case where the acceleration control of the photosensitive drum is completed and switches to the constant-speed control immediately after the rotation position mark 1121 passes the rotation reference position sensor 1120, the formation of an image cannot be started until the rotation position mark 1121 passes the rotation reference position sensor 1120 next time. Namely, the output of an image delays about one second at the maximum, and this causes considerable deterioration of performance for printers assuming correction.
Here, if the time period required for stabilization of the rotation speed of the polygon mirror is longer than the time period required for stabilization of the rotation speed of the photosensitive drum, the formation of an image cannot be started until the rotation speed of the polygon mirror stabilizes, and hence the above described problem is alleviated or does not arise. However, because polygon mirrors of recent years have been increasingly reduced in weight, and the rotation speed of the polygon mirror stabilizes within a short time period than the time period required for stabilization of the rotation speed of the photosensitive drum. For this reason, in response to stabilization of the rotation speed of the photosensitive drum, image forming apparatuses of recent years go into a state of readiness to carry out image formation. Thus, the above described problem may arise.